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Mouthwash and oral cancer

14 January, 2009

“Mouthwash ‘can cause oral cancer,’” reported The Daily Telegraph. The newspaper said that researchers have claimed there is now ‘sufficient evidence’ that mouthwash containing alcohol contributes to an increased risk of the disease.

The newspaper said the claims follow a review of the latest studies, with the authors suggesting that mouthwash should be “taken off supermarket shelves and labelled with health warnings”.

The authors of this review have presented their own subjective opinion about the evidence for harms associated with alcohol-based mouthwash. Importantly, they highlight the fact that some studies have found no link between these mouthwashes and oral cancer. This confirms a difference of opinion that requires further investigation.

Given that alcohol consumption has been linked to an increased risk of oral cancer, it is important that more research is carried out using systematic, robust methods.

Where did the story come from?

Drs MJ McCullough and CS Farah carried out this study. It is unclear whether any external funding was received. The study was published in the peer-reviewed medical journal, the Australian Dental Journal.

What kind of scientific study was this?

The publication is a non-systematic, narrative review of the evidence linking mouth cancer with the use of alcohol-based mouthwashes. It also discusses the results from laboratory and animal studies. The authors put forward mechanisms for a possible increase in risk before making their subjective conclusions about the balance of evidence.

What were the results of the study?

The authors begin by discussing the worldwide incidence of oral cancer and current survival rates. They list long-established risk factors for the development of oral cancer, including smoking, alcohol consumption and chewing tobacco, as well as other suspected factors, such as viruses, diet and poor oral hygiene.

The researchers then go on to discuss some of the evidence, including epidemiological (population) studies that have found a link between drinking alcohol and mouth cancer. They discuss several laboratory and animal studies that have investigated the effects of alcohol on cells and tissues in cultures and in animals. Based on this, the researchers put forward a proposed mechanism for the possible effects.

They then talk about studies that have looked specifically at the effects of alcohol-based mouthwashes on oral health in humans. They say that, while some case-control studies have found an association between alcohol-containing mouthwashes and oral cancer, others have not. They focus on the results of a recent and large case-control study in 6,000 people (3,200 with head and neck cancers and 2,752 controls), which found that the use of mouthwash increased the odds of oral cancer by nine times in current smokers. They do not provide results for non-smokers or former smokers.

What interpretations did the researchers draw from these results?

The researchers say that on the basis of their review, they “believe that that there is now sufficient evidence to accept the proposition that alcohol-containing mouthwashes contribute to the increased risk of development of oral cancer”. They also say that they believe that healthcare professionals should not recommend the long-term use of alcohol-based mouthwashes.

What does the NHS Knowledge Service make of this study?

This narrative review collected research on alcohol consumption, along with the use of alcohol-based mouthwashes and oral cancer. The review then discussed the possible ways that these may increase risk of the disease.

There are several points to highlight:

This appraisal cannot comment on the quality of the studies in the review because it has not assessed them.

It is not clear how the authors identified the studies they chose to discuss, which their opinions are based on. Possible inaccuracies in these studies may include the self-reporting of mouthwash use, duration of use, types of mouthwash and alcohol content, and also whether possible confounding factors have been taken into account.

The discussion of evidence here is a considered and important one, and the authors do highlight the fact that some studies have found no link at all.

A more robust way to review the balance of evidence would be to adopt an approach in which all the research is identified through explicit methods and systematically appraised. Only through this method is it possible to ensure that studies with positive and negative results are all included and given the same weighting.

Importantly, the main study that the researchers discuss is a case-control study. This type of study cannot prove the causes behind oral cancer. The authors report the increased risk of oral cancer with alcohol-containing mouthwash among current smokers only (who are at increased risk of oral cancer). They do not provide results for former or non-smokers. This is important because smoking is the most well-established risk factor for oral cancer.

Given that drinking alcohol is the second most important risk factor for oral cancer (source: Cancer Research UK), this is an important discussion, and more research into this is needed.